Residual Images.
We follow LMNet [1]} using residual images to exploit the spatial-temporal information from sequential LiDAR scans.
To generate a residual image between the current frame \(l\)  and the previous frame \(k\) , there are three steps. First, using the relative pose to transform the previous scans \(k\)  to the current coordinate system. Second, re-projecting the transformed past LiDAR points into range image. Third, computing the residual \(d^{l}_{k,i}\)  for each pixel \(i\)  by computing the normalized absolute difference between the ranges of the current frame \(l\)  and the transformed frame \(k\)  as
\(d^{l}_{k,i} &={\left\lbrace \begin{array}{ll}{|r_i - r^{k \rightarrow l}_i|}/{r_i} & \text{$i \in $ valid pixels,} \\0 & \text{otherwise,}\end{array}\right.}\) 
